It is frequently necessary to protect such junctions against the environment in order that the cables or other substrates may continue to function properly. Protection generally has to be provided against moisture, corrosive chemicals as well as insect and animal damage etc. The intention when enclosing a junction such as a cable splice is to make good the original cable insulation that had to be removed in order to connect the conductors, and it is generally required that the life-time of the seal provided by the new enclosure be comparable to that of the original cable insulation. It will be appreciated therefore that the material of the enclosure must provide a highly resistant barrier for a considerable period of time.
One way of providing such a barrier is to install around the cables a splice case comprising an imperforate sleeve of a modified polyolefinic material in conjunction with a high performance adhesive. Such sleeves are conveniently produced by extruding a continuum of material. The sleeve is preferably made recoverable so that it can be shrunk (or otherwise recovered) into close contact with the cables.
There is a further consideration relevant to the design of enclosures for cable splices, and that is the ability to retain pressure. Many types of cables and splice cases are pressurised during use, are assessed in terms of pressure retention to determine their quality, or become subject to incidental pressurisation during use. The importance of this consideration is of course different in each of these three situations, but it is accepted that the ability to retain some degree of pressure is a necessary feature of a splice case if environmental protection is to be achieved.
The most stringent requirements are for a splice case for pressurised cables, such as main cables in a telecommunications system. These cables are pressurised to prevent ingress of water in the event of damage and to provide a means of fault detection. Here the product must withstand a pressure of the order of 10 psi (70 kPa) throughout its life, and a functional test designed to mirror such long term performance requires impermeability at, say, 70 kPa over 10 eight hour cycles between -40.degree. C. and +60.degree. C. in air (Bell cycle). An alternative cycle is in water over four hours at 105 kPa between 5.degree. and 50.degree. C. In addition to this cyclical environment test, the product may be tested for integrity by pressurisation at 150 kPa in water for about 15 minutes at 23.degree. C. No leak should be observable. A product that is to operate continuously at pressure should also possess long term creep resistance if it is not to become significantly distorted during use.
In telecommunications distribution cables, for example, an ability to retain pressure is required as an indication of completeness of environmental sealing, although the cables are not pressurised during use. Various temperature/pressure cycles have been devised for this purpose, and one that is preferred is a modified Bell Cycle which involves temperature variation from -40.degree. to 60.degree. C. over 8 hours at an air pressure of 40 kPa. The splice case should show no leak after 10 cycles. An alternative cycle is a temperature variation between room temperature and 70.degree. C. at a pressure of 105 kPa over 4 hours.
These and other cable splice cases may become pressurised through being exposed to sunlight, or through the heat involved in the last stages of heat recovery when the seals to the cable have been formed. In such cases it is necessary that the splice case be able to maintain this temporary, and generally rather low, pressure if the environmental seal is not to fail.
UK patent application No. 2135836 discloses a splice case or other hollow pressure vessel capable of pressure retention and made from a recoverable fabric. Such a splice case comprises a recoverable sleeve that can be wrapped around a cable splice and shrunk to seal to the cable. Installation is simple and the product is reliable even under unfavourable conditions.
Specific designs disclosed in the prior art may not be ideal for cheap, rapid manufacture and may not have sufficiently high recovery ratios for some uses.